The present invention generally relates to contactor circuits. More specifically, to rapidly dissipating energy from an energized contactor coil.
Electronic coil designs may utilize a dual wound DC coil that pulls an armature into the picked up, i.e., closed, position and holds the armature in the closed position. The first coil is called the pickup, or inrush coil. It is a low impedance, high current coil that generates the large amount of flux, NI, necessary to pull the armature in from the dropped out, i.e., open, position. The large amount of wattage that the pickup coil generates can thermally damage the electronic coil assembly if the pickup coil were allowed to remain on continuously. It is the primary job of the contactor""s electronic circuit to turn the pickup coil on until the armature is fully pulled in, and then to turn the pickup coil off. The pickup coil has a large L/R ratio, or time constant, due to the many turns of wire and small impedance of the coil.
The second coil or holding coil, is a high impedance, low current coil that generates the smaller amount of flux necessary to hold the armature in the closed position once the pickup coil is de-energized. The holding coil typically incorporates a smaller gauge wire than used in the pickup coil. The smaller wire provides a higher resistance value than present in the pickup coil. Therefore, the holding coil has a smaller L/R ratio and time constant than the pickup coil.
Dropout or opening of the contactor""s armature during normal operation is typically accomplished by interrupting the power supply to the coil circuit and allowing the energy within the holding coil to naturally decay through the holding coil""s impedance. In large contactors, the amount of energy required to decay before the contactor dropouts can be rather large. Since the impedance of a large contactor can be small, it may take a long time, i.e., hundreds of milliseconds, for the energy within the coil to decay before the contactor dropouts.
Another mode of operation for the contactor is called jogging. Jogging occurs when the contactor is picked up for a very short duration and then dropped out again. The jogging cycle may be repeated many times over a brief period. Jogging is often used in positioning booms or heavy duty cranes. Due to the fact that the pickup coil has a long time constant, the pickup coil may still be energized during repeated attempts to interrupt the power supplied to the pickup coil. The failure to sufficiently dissipate the energy stored in the contactor""s coil will interfere with the jogging operation. Therefore, the energy stored in the pickup coil must be dissipated rapidly to force a fast dropout so as not to interfere with the jogging operation and to ensure accurate movement of apparatuses controlled by the contactor circuit.
Electrical components such as Zener diodes, gas tubes or capacitors have been frequently used to repetitively dissipate the energy from electrical components. The problem with using diodes and capacitors to dissipate contactor coils is the amount of energy stored in the coil relative to the dissipation capability of the diode or capacitor. For instance, a capacitor having the proper voltage handling capability and sufficient capacitance to dissipate the energy of the pickup coil would need to be very large. With physical space of the printed circuit board at a premium, a physically large capacitor would not be acceptable. Zener diodes may work on small contactors, but Zener diodes capable of working at greater than 100V are limited. Finding a Zener diode with the ability to dissipate many amperes is virtually impossible unless the Zener diode is so physically large that it would be impractical to use on a small printed circuit board. Gas tubes exhibit follow currents and are not the best candidate for this application.
Prior to the present invention, a need existed to rapidly and repetitively dissipate the stored energy in the coil of a contactor after the contactor has been commanded to dropout. Furthermore, the method used should be less costly to existing procedures and adaptable for use in smaller sized circuits.
This invention is designed to resolve these and other problems.
A metal oxide varistor (MOV) is capable of rapidly absorbing energy from a coil of a dropped out contactor. Typically, MOVs have been used in xe2x80x9cone shotxe2x80x9d or single use applications such as power surge protection circuits. However, MOVs are capable of being implemented in repetitive applications requiring rapid power dissipation such as in the jogging operation of a contactor. Moreover, the ratio of physical size to dissipating capability of the MOV and its relative inexpensiveness as compared to other xe2x80x9csimilarxe2x80x9d dissipating techniques, qualify the MOV as an excellent choice for use in rapidly dissipating contactor coils.
Accordingly, the first embodiment of the present invention is directed to a method of dissipating energy from a contactor controlled by a switching device, the steps include: operably connecting a metal oxide varistor to the switching device; interrupting electrical current to the contactor; absorbing energy from the contactor; and, limiting the amount of voltage across the switching device.
Another embodiment of the present invention is directed to a method of dissipating energy from a dual wound coil controlled by a switching device, the steps include operably connecting a metal oxide varistor to the switching device; interrupting electrical current to the pickup coil of the dual wound coil; absorbing energy from the pickup coil; and, limiting the amount of voltage across the switching device.
A further aspect of the embodiments above includes an insulated gate bipolar transistor (IGBT) for the switching device.